1. Field of the Invention
This invention relates generally to pressure relief valves adapted for low pressure operation, and more particularly to such pressure relief valves including a fluid pressure responsive actuator for the main valve member and having a dome chamber adjacent the movable pressure responsive member of the actuator.
2. Description of the Prior Art
Heretofore, pilot operated safety relief valves have been provided for pressure vessels, such as tanks or main flow lines, with the pressure relief valve having a main valve member normally seated on an inlet seat and a diaphragm actuator connected to the main valve member for movement therewith. A dome chamber is provided over the diaphragm and is in fluid communication with a pilot valve which senses the pressure at the valve inlet which is in fluid communication with the pressure vessel. The pilot valve may be set for actuation at a predetermined inlet pressure to effect bleeding of fluid from the dome chamber for opening of the main pressure relief member under line or inlet pressure. The main valve member usually comprises a valve disc or plate mounted on an inlet seat and is often of a substantial weight. Upon reduction of fluid pressure within the dome chamber by the exhaust of fluid therefrom by the pilot valve, the pressure at the inlet overcomes the weight of the main valve member and the friction of the moving members to unseat and open the main valve member for pressure relief.
The pilot operated relief valve includes a self-contained fluid system which does not require any external power or pressure source. The pilot valve, using the associated fluid medium and pressure, automatically controls the actuator pressure to either open or close the main valve depending on the pressure setting of the pilot relative to the actual system pressures. Under normal system or line operating conditions, the same pressure is acting downwards against the actuator and upwards against the main valve member. Since the actuator has a larger effective area than the main valve member, the net force is downward which urges the main valve member against the seat and thus keep the main valve member closed. While the pilot and main valve are closed, there is no bleed to the atmosphere.
When the system pressure rises to the pilot set point due to an overpressure condition, the upward force in the pilot sense chamber overcomes the downward spring force to open the pilot seat to allow flow through the pilot and out to the atmosphere. When the pressure in the dome chamber decreases to a point where the upward force on the main valve member is greater than the downward force of the actuator, the main valve opens. After the excess pressure has been relieved and the system pressure is again below the set point of the pilot, the main valve member will return to its normal closed position. In the normal closed position of a diaphragm actuator, the inner or underside of the diaphragm opposite the dome chamber is exposed to fluid pressure from the valve body chamber which is normally under ambient pressure conditions.
In a piston type actuator, the minimum pressure differential between dome pressure and inlet pressure is around 5-15 psi resulting primarily from the weight of the piston. With a bellows or diaphragm type actuator a pressure differential of around 1/3 psi is normally required for opening of the main valve member. For many applications today, particularly where environmental concerns only permit minimal emissions, it is desirable that a relief valve be responsive to opening fully at much lower pressures than heretofore. It is desirable that a pilot operated safety relief valve be set and open fully at very low pressure (i.e. 1-10 inches WC). The fluid pressure actuator for a safety relief valve while commonly employing a diaphragm as the pressure responsive member, may also comprise a piston or a bellows.
Present code requirements for a low pressure vessel do not permit the setting of a valve above the maximum allowable working pressure (MAWP), yet many safety relief valves do not fully open until a pressure beyond the MAWP is reached. Thus, a more effective safety relief valve is needed for full opening at a very low pressure differential.